1. Field of the Invention
The present invention relates to a display device and a method of calculating a correction value for correcting an image signal supplied to a display panel in a display device.
2. Description of the Related Art
As can be seen from Japanese Unexamined Patent Application Publication No. 2005-195832, for the purpose of correcting unevenness of luminance and chromaticity of a display device (or simply a display panel) so as to improve uniformity, an unevenness correcting device for determining a correction value by a coordinate of an X direction, a Y direction and a grayscale direction (Z direction) of a panel, which is called 3D-γ system, has been put to practical use.
The unevenness correcting device is mounted in an image display device such as a television device as a circuit unit for performing a correcting process with respect to an image signal supplied to a display panel unit.
FIG. 24 shows an example of signal correction using an unevenness correcting circuit. This is a 2D map diagram of a luminance-corrected image to be output when a uniform luminance image is input to a display panel.
For example, it is assumed that an image signal value (grayscale value) is represented by 10 bits and grayscale has 1024 steps of 0 to 1023. If image signals having a grayscale value of “512” are given to an overall screen, that is, all pixels constituting a screen, the overall screen should display a uniform image having a grayscale value of “512”. However, due to luminance unevenness of the display panel, a darker portion or a brighter portion than the portion having the grayscale value of 512 is generated on the screen. Thus, the uniformity of the screen is low. In order to improve this, the image signal values given to the pixels are corrected according to the characteristics of the luminance unevenness.
That is, a signal for the portion of a low-luminance on an unadjusted panel is converted to an image signal having a high-luminance value, a signal for the portion of a high-luminance on the unadjusted panel is converted to an image signal having a low-luminance value, and these signals are given to the display panel as the corrected image signals, thereby outputting a desired image having uniform luminance.
For example, an image signal value corrected to have a grayscale value higher than “512” is given to a pixel of a darker portion than “512”, even when the grayscale value “512” is given, on the screen depending on a luminance difference.
In addition, an image signal value corrected to have a grayscale value lower than “512” is given to a pixel of a brighter portion than “512”, even when the grayscale value “512” is given, on the screen depending on a luminance difference.
FIG. 24 shows grayscale values as the correction values on an XY plane corresponding to a screen plane and shows corrected grayscale values by the shades of the pixels. By such correction, it is possible to prevent deterioration of uniformity due to luminance unevenness characteristics of the display panel and to display an image with a high quality.
In an unevenness correcting circuit of a 3D-γ system, such a 2D map is prepared for a uniform image having a variety of luminance values.
FIG. 25 shows an input/output function of panel luminance correction by making a graph of the Z direction (grayscale direction) of the 3D-γ system.
If the panel is completely uniform, a linear graph representing the output of an input signal without modification is obtained. However, the graph of FIG. 25 shows that the actual input/output function has variations in order to correct the uniformity on a pixel-by-pixel basis.
For example, in a grayscale value Ain of an input side (horizontal axis), an output side (vertical axis) as a corrected grayscale value is in a range from Aout1 to Aout2. When an image signal with the grayscale value Ain is given to all pixels such that a uniform image is displayed, the grayscale value is necessary to be corrected for each pixel in order to actually display the uniform image. As a result, the correction value of each pixel is in a range from Aout1 to Aout2.
The range of the correction value is different for each grayscale value. Due to the variation of each grayscale value, the 2D map is necessary to be prepared for each grayscale value.
The unevenness correcting circuit includes a lookup table unit 100 and a correction operation circuit 101 as shown in FIG. 26.
In the lookup table unit 100, a lookup table as the 2D map is stored for each grayscale value. In each lookup table, with respect to input grayscale values, a grayscale value (or a coefficient for obtaining a corrected grayscale value) as a correction value is stored for each pixel.
The correction operation circuit 101 reads values necessary for an operation from the lookup table unit 100, and computes and outputs image signal values for correcting luminance unevenness and chromaticity unevenness of a panel using the values, with respect to input original image signal values.
In order to retain unevenness correction data with respect to all the X direction, the Y direction and the Z direction, the amount of data may be impractically enormous. Therefore, a method of storing correction values with the 2D maps for the representative Z coordinate (grayscale value) and estimating and using correction values from the representative correction values in the other coordinates is generally applied.
For example, although the grayscale values of the 1024 steps of “0” to “1023” are considered as the grayscale value (Z direction) in FIG. 25, it is not practical that a 3D-γ system is established by retaining 1024 2D maps (lookup tables).
Therefore, of the values from “0” to “1023”, n representative input values obtained by sampling several correction values, such as “0”, “64”, “128”, . . . , and “1023”, in the Z direction are set and n lookup tables for the n representative input values are retained.
If the input image signal value is a grayscale value which is not sampled, an interpolation operation is performed using correction values stored in the lookup tables of the grayscale values that are larger and smaller, respectively, than the input image signal value and closest to this input image signal value. For example, correction values are obtained by a linear interpolation operation.
In such a correction system, how the correction values of the pixels are determined will be described.
In FIG. 27A, a horizontal axis denotes a position X of any horizontal line of an uncorrected panel and a vertical axis denotes the luminance of the position. Panel luminance LP when a certain grayscale value V is input is denoted by a solid line. It can be seen that the panel luminance is not uniform due to unevenness. In addition, panel luminance LP is luminance which actually appears on the panel when one grayscale value V is given to all the pixels of the panel.
In addition, there is a tendency that the luminance of the central portion of the panel is highest.
In order to calculate a correction value for correcting an input image signal with respect to a panel with unevenness, in an existing method, target luminance values of all pixels are set to target luminance TG denoted by a dotted line in FIG. 27A.
That is, if a grayscale value V is given and the pixels emit light with luminance Lt, originally, the luminance of the overall screen uniformly becomes luminance Lt. With respect to the overall screen (all pixels), the target luminance becomes TG=Lt.
Next, correction values for the pixels are obtained such that all the pixels have a target luminance value (luminance Lt).
In FIG. 27B, a horizontal axis denotes grayscale V and a vertical axis denotes luminance L. An ideal V-L curve has target luminance Lt when the grayscale is V.
Meanwhile, a V-L curve before correcting the luminance of a certain pixel to be corrected is positioned below the ideal V-L curve, as shown in FIG. 27B. Then, in order to output the target luminance Lt, (V+ΔV) is necessary as a grayscale value given to the pixel.
That is, it can be seen that (V+ΔV) is necessary to be output when V is input to the unevenness correcting circuit.
As shown in FIG. 28A, a graph made by obtaining all the correction values (V+ΔV) satisfying such a condition in the X direction of the panel is denoted by a solid line H indicating the correction values. As the characteristics of the elements of the panel, a small correction value is obtained at a position having a high luminance and a large correction value is obtained at a position having a low luminance.
In addition, the unevenness correcting circuit is necessary to satisfy the above-described function with respect to all input grayscale.